Cider (MSDN TV demo here) is the visual designer for the Windows Presentation Foundation (used to be Avalon), part of WinFX, and it was just released for the first time as part of the WinFX December 2005 CTP.

It works just fine in VS 2005 RTM, which is how I tried it. My impressions are:

• It has a lot of bugs. They are everywhere, but that's to be expected.
• It can nevertheless do some serious work for you if you want to create XAML - certainly much faster than writing the XML yourself.
• It provides a quick and easy look at some of the controls that are in WPF. Without the visual designer, these could only be found by reading docs.
• It shows off some of the fantastic functionality in WPF, like the fact that UIs are resolution independent. Just drag the zoom slider in the designer pane and see your UI being resized live!